A “digital subscriber loop”(“DSL”) is a type of communications connection and/or service which is now being offered by many local exchange carriers (e.g., telephone companies) to consumers and businesses alike as a way of receiving faster Internet connections and downloads.
When a signal δ(n), where “n” represents an increment of time, is fed into one end of a DSL channel, a scaled (i.e., amplified or attenuated) version of δ(n) and scaled versions of delayed replicas (i.e., echo) of δ(n) are received at the opposite end of the channel. This characteristic is known as the “memory effect” of the channel. The signal received at the opposite end of the channel is known as the impulse response h(n). Individual components of h(n) determine how much each delayed replica of δ(n) is amplified or attenuated.
Discrete multi-tone (“DMT”) transceivers are devices which are designed to send and receive DSL-compatible signals (hereafter “DSL signals”) over DSL channels. The quality of a DSL signal received by a DMT transceiver depends on a number of factors. One such factor is the amount of interblock interference (“IBI”). IBI is caused by the aforementioned memory effect. In particular, IBI results from portions of past information “blocks” leaking into current information blocks of a signal δ(n). DMT transceivers are designed to generate and insert a guard time sequence “M”, called a “cyclic prefix”, between each transmitted information block in an attempt to reduce the undesirable effects of IBI. The hope is that the information from previous blocks will die out during this time sequence before it leaks into a current block. In order to completely eliminate IBI, the cyclic prefix M must be at least as long as the length “L” of the impulse response. However, this can rarely be achieved in practice because such a long cyclic prefix M would significantly decrease the throughput of a channel. Thus, it is necessary to use supplemental methods and devices to reduce IBI.
FIG. 1 depicts a graph of a typical, simplified impulse response h(n), of a DSL channel. The impulse response h(n) and its associated length “L” may be expressed as:                                           h            ⁡                          (              n              )                                =                                    ∑                              -                                  L                  2                                                            L                1                                      ⁢                                          h                k                            ⁢                              δ                ⁡                                  (                                      n                    -                    k                                    )                                                                    ;                  L          =                                    L              1                        +                          L              2                        +            1                                              (        1        )            
Impulse response h(n) comprises a causal portion “L1” and a non-causal portion “L2”. The causal portion L1 consists of the real-time portion of the impulse response h(n). The non-causal portion L2 consists of the delayed, or stored, portion of the impulse response h(n). The impulse response h(n) can be arbitrarily partitioned into three segments, namely: an “upper tail” ha(n); a “lower tail” hb(n); and a “main lobe” hc(n). The segments are defined as follows:ha(n)=h(n+M+1) for n 0; ha(n)=0 otherwisehb(n)=h(−n−b 1) for n 0; hb(n)=0 otherwisehc(n)=h(n) for 0 n M; hc(n)=0 otherwiseThe main lobe hc(n) comprises the segment of the impulse response h(n) which falls within the time range of the cyclic prefix M. The location of h(0) is the starting location of the main lobe hc(n). This location is called the time of reference (“TOR”). The tails ha(n) and hb(n) fall outside of the time range of the cyclic prefix M. Since the tails ha(n) and hb(n) are the only segments of the impulse response h(n) which fall outside of the range of the cyclic prefix M, they are the only segments that contribute to IBI.
A known method for reducing IBI involves computing the TOR that maximizes the main lobe energy “Ec”, which is the energy under main lobe hc(n). The main lobe energy Ec is given by the following equation:                              E          c                =                              ∑                          n              =              0                        M                    ⁢                                    h              ⁡                              (                n                )                                      2                                              (        2        )            
According to this method, maximizing the main lobe energy Ec is achieved using a window correlation technique which correlates h2(n) with a rectangular windowing function w1(n) of size “M+1” to generate a cross-correlation function and which computes a TOR. Using this technique, the location of the TOR corresponds to the maximum output value of the cross-correlation function. The windowing function w1(n) is shown in FIG. 2.
Computing the time of reference based on maximizing the main lobe energy Ec is similar to computing the time of reference such that the energy under the tails ha(n) and hb(n) is minimized. This approach is intuitive. Because the tails ha(n) and hb(n) alone contribute to IBI, it seems to follow that the impulse response h(n) should be partitioned such that the tails have as little energy as possible. However, this approach erroneously assumes that individual points along the tails ha(n) and hb(n) contribute a uniform amount of IBI power. Therefore, although this approach reduces IBI, it does not provide the optimum TOR such that IBI is minimized.
The present inventor has discovered that, contrary to previous belief, that each point along the tails ha(n) and hb(n) contributes a non-uniform amount of IBI power. More specifically, the present inventor discovered that IBI power increases linearly from point to point, as the distance from the point to the main lobe hc(n) increases. Accordingly, it is desirable to provide methods and devices for computing the optimum time of reference so that IBI is minimized.
Other desires will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings and claims.